The present invention relates to Mach-Zehnder filters and more particularly to Mach-Zehnder filters that are characterized by narrow peaks and wide separation between peaks.
There is an emerging need for narrowband filters. Such devices are needed in the 1550 nm window for modifying the Gain spectrum of erbium fiber amplifiers. They will also be widely used in trunk lines as well as in fiber-to-the-subscriber architectures.
There is a need for both wavelength tunable components and fixed wavelength components. In an all optical network, for example, the device can be tuned at the receiver end in order to detect the desired incoming signal. In a second approach, tunable lasers are used to send a plurality of signals, and the desired signal is detected by employing a receiver having a fixed filter. The transmission system could also employ both fixed lasers and filters. The wavelength separation capabilities of such filters needs to be on the order of tens of nanometers to as small as a nanometer. Moreover, these components will need to be environmentally stable and very reliable.
The Mach-Zehnder filter is known for its narrowband wavelength capabilities. It has been proposed that filters having pass bands as narrow as 1 nm be formed by connecting two evanescent couplers with unequal fiber lengths between them. See OFC Conference on Optical Fiber Communication, Minitutorial Sessions, Jan. 22-26, 1990, page 256 (part of a presentation on "Dense WDM Techniques" by C. A. Brackett), and P. E. Green, Fiber Optic Networks, Prentice Hall, 1993, pp. 123.
FIG. 1 shows a schematic diagram of a conventional Mach-Zehnder filter. Two couplers C.sub.1 and C.sub.2 are concatenated by optical waveguide paths or fibers F.sub.1 and F.sub.2. For the sake of simplicity, optical fibes will usually be discussed. The couplers for this conventional Mach-Zehnder device, which are typically evanescent type couplers, are usually 3 dB couplers, whereby the optical power that is applied to input port 2, for example, is equally divided between the two outputs of coupler C.sub.1. Mach-Zehnder devices can also employ non-evanescent planar couplers; see S. I. Najafi, Introduction to Glass Integrated Optics, Artech House, 1992, pp. 156-160. For certain types of filters, one or more of the couplers can unequally split the input power. One of the fibers has means OPLD to impart to it an optical path length difference so that there is a phase shift between the two input signals that are applied to coupler C.sub.2. A phase shift can be induced by employing fibers having different lengths or different refractive index profiles, or by inserting phase shifting means into one of the waveguide paths.
The power output at port 3 can be written as EQU P=cos.sup.2 (.pi..multidot..DELTA.L.sub.P /.lambda.) (1)
where .DELTA.L.sub.P is the optical path length difference (OPLD) between the paths connecting the two couplers. Therefore, the device response is periodic function of inverse wavelength, and the output power spectrum thereof is similar to that illustrated in FIG. 2. The wavelength separation between the peaks 12 in FIG. 2 would be halved if .DELTA.L.sub.P were doubled.
Two commonly employed techniques for achieving an OPLD are (1) providing connecting fibers F.sub.1 and F.sub.2 having different lengths, and/or (2) providing connecting fibers F.sub.1 and F.sub.2 that propagate light at different speeds, usually by providing fibers with different cores. For case (1) where identical fibers have different lengths, equation (1) becomes EQU P=cos.sup.2 (.pi..multidot.n.DELTA.L/.lambda.) (2)
where n is the refractive index of the fiber cores, and .DELTA.L is the difference in the lengths of the fibers in the phase shift region that interconnects the couplers C.sub.1 and C.sub.2. The optical path length of an optical fiber can be changed by heating, bending, stretching or the like. The optical path length of a planar optical waveguide path formed of electro-optic material can be changed by applying an electric field to it.
For case (2), where connecting fibers F.sub.1 and F.sub.2 have different core refractive indices, equation (1) becomes EQU P=cos.sup.2 (.pi..multidot.L.multidot..DELTA.n/.lambda.) (3)
where L is the length of fibers F.sub.1 and F.sub.2 in the phase shift region that interconnects the couplers C.sub.1 and C.sub.2, and .DELTA.n is proportional to the refractive index difference between the two fiber cores and is approximately equal to f.multidot.n.sub.2 (.DELTA..sub.2 -.DELTA..sub.1), where .DELTA..sub.1 and .DELTA..sub.2 are the .DELTA..sub.1-2 values of fibers F.sub.1 and F.sub.2, respectively. The term .DELTA..sub.1-2 is the relative refractive index difference between the core and cladding of a given fiber and is equal to (n.sub.1.sup.2 -n.sub.2.sup.2)/(2n.sub.1.sup.2), n.sub.1 and n.sub.2 being the fiber core and cladding refractive indices, respectively. The factor f takes into account the fact that the phase shift is proportional to the so-called "effective" refractive index which depends on both the fiber .DELTA..sub.1-2 value as well as the core diameter. Equation (3) then becomes EQU P=cos.sup.2 (.pi..multidot.L.multidot.f.multidot.n.sub.2 (.DELTA..sub.2 -.DELTA..sub.2)/.lambda.) (4)
where n.sub.1 is the core refractive index and the value of f.multidot.n.sub.1 can be taken to be approximately 1 if the core diameters are essentially equal. Equation 4 is plotted in FIG. 2, for a single-stage Mach-Zehnder filter in which fiber F.sub.1 has a .DELTA..sub.1-2 value of 0.3% and fiber F.sub.2 has a .DELTA..sub.1-2 value of 1.0%, the optical path length L (the length of each of the fibers F.sub.1 and F.sub.2 between couplers C.sub.1 and C.sub.2) being 2 cm.
Mach-Zehnder devices in which an optical path length difference is obtained by employing two concatenating fibers F.sub.1 and F.sub.2 of identical length but different core refractive indices are taught in U.S. patent application Ser. No. 08/038,244 (W. J. Miller et al. 10-13) "Monolithic Mach-Zehnder Device" filed Mar. 29, 1993, now U.S. Pat. No. 5,295,205, and the publication B. Malo et al. "Unbalanced Dissimilar-Fibre Mach-Zehnder Interferometer: Application as a Filter" Electronics Letters, 12th Oct. 1989, Vol. 25, No. 21, pp. 1416-1417.
If multiple devices are serially cascaded, as shown in FIG. 3, the total response is simply a product of terms like those in equations (1), (3) or (4). If the optical path length difference of one of the devices is chosen such that it is a multiple of the other, e.g. if OPLD-2 is a multiple of OPLD-1, a narrow band filter response results. Every other peak remains, the intervening wavelengths between peaks being substantially eliminated, as shown in FIG. 4, which is a graph of the calculated output power for a two-fiber, two-stage Mach-Zehnder device in which .DELTA..sub.1 is 0.003, .DELTA..sub.2 is 0.01, L.sub.1 is 1 cm and L.sub.2 is 2 cm. The cascading of additional Mach-Zehnder devices increases the wavelength separation between peaks. The sharpness of the peaks is controlled by the Mach-Zehnder with the largest optical path length difference. Therefore a larger OPLD provides a sharper peak. The distance between peaks is a function of the number of cascaded devices and the relative optical path length differences in each stage. The improvement in filtering properties achieved by the device illustrated in FIG. 3 as compared with that illustrated in FIG. 1, i.e. the improvement in the filtering properties illustrated in FIG. 4 as compared with the properties illustrated in FIG. 2, is known as "finesse" which is defined as the ratio of the wavelength separation between adjacent peaks to the peak widths. Greater finesse is achieved as the number of cascaded Mach-Zehnder devices is increased.
Mach-Zehnder filters of the type shown in FIGS. 1 and 3 are tunable. The peaks are shifted by changing the OPLD's. Therefore, such a device could be used to tune to various optical wavelengths as radio receiver is tuned to radio wavelengths.